It is known in the art relating to mechanical limiting speed governors to provide a fluid actuated piston, such as an air piston, operative as a part of the governor limiting speed setting means to modify the established limiting speed upon application or variation of air pressure acting against the piston.
As an example, U.S. Pat. No. 2,771,788 Frick and HIckson shows in FIGS. 2 and 5 a speed setting arrangement in which the compression of the high speed spring 111 is varied by the movement of an air piston 105 in response to the application of air pressure against the piston under predetermined conditions, thus varying the governor limiting speed.
Another arrangement is shown in U.S. Pat. No. 2,656,174 Crookston, wherein an overspeed spring 54 and an air bias piston 66 are arranged to act in parallel fashion against the speed related force generated by the flyweights 17' of the governor. These devices, acting together with a hydraulic dashpot piston 68, are combined to provide smooth speed setting operation over a relatively wide range of speeds.
In still another arrangement, shown in applicant's U.S. Pat. No. 4,082,074 which is assigned to the assignee of the present invention, an air biased piston 92 is arranged to act in a direction supplementing the force of the speed responsive flyweights to thus provide a reduction in the governor limiting speed setting in proportion to the pressure of air applied against the piston 92.
These prior art arrangements utilize the application of fluid pressure to a piston in various ways to modify the limiting speed setting of a mechanical engine governor. However, they all combine the fluid piston speed modifying means with a conventional spring to provide at least a portion of the speed setting force that acts against the speed responsive flyweights, or similar mechanism. The biasing force thus created establishes the limiting speed setting which is matched by the speed related force of the flyweights at the point of governor balance. The use of such biasing springs in mechanical engine governors in which the speed balanced condition may occur at various positions of spring compression inherently introduces a certain amount of speed droop, or limiting speed variation with load, into the system. In addition, some degree of friction is provided which creates hysteresis but has the advantage of damping oscillations of the mechanism, thus helping to provide a stable speed control system.
While a certain amount of speed droop and hysteresis is acceptable for most engine speed governing functions, these characteristics are not so desirable when it is desired to use the governor for some other related purposes. For example, the speed droop characteristic inherent in the governor arrangement of the previously mentioned U.S. Pat. No. 4,082,074 causes a variation in the controlled speed under varying load conditions, which is undesirable when the engine governor is used as a vehicle speed control, one of the uses of the governor described in the patent. This speed variation is, however, quite acceptable when the governor is performing its primary function of engine speed control or while it is used to control the engine speed for an auxiliary drive system.